doc(security): amend outdated security policy info

[eternal-flame-AD]

Added more guidance to clear up some friction points regarding vulnerability report submissions.

The editorial goals are:

• Provide more clarity as to what are supported versions, and provide actionable alternatives for findings that do not apply
• Reflect that we are now talking reports via Github Security.
• Nudge reporters to stick to the CVE vocabulary so it's easy to read and reduces paperwork.
• State expectations and goals of our advisory program in a soft but informative manner.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.34%. Comparing base (ed8af76) to head (2a485c9).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #993   +/-   ##
=======================================
  Coverage   74.34%   74.34%           
=======================================
  Files          66       66           
  Lines        3461     3461           
=======================================
  Hits         2573     2573           
  Misses        688      688           
  Partials      200      200           

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